If you know how to write HTML & CSS, you might think writing a responsive HTML email would be simple. WRONG !!! There is reason why such services exists. How to make sure your email look the same on ALL email clients is the first & most difficult problem you will run into. Have a look at this test page provided by Email on Acid

I attempted to write a simple responsive HTML email with the following feature:

Have an banner image on the top

The rest of the body is just text

I found this tutorial and followed it, the first thing I noticed is that:

Everything should be inside a table tag

Because mail clients like outlook would ignore a few important CSS properties: background-image, float, margin and display.reference here

Validate your HTML using W3C's validator

How to send HTML mail using Mac's Mail app?

Open your HTML file locally in Safari

Press 'command + i' (now the Mail app should pop up with the HTML content in it)

Send it

DO NOT use 'command + a' to select the entire HTML page and copy and paste it into the Mail app. This works, but during this process, certain css attribute will be changed and you are likely to lose the responsiveness of your site!

This is only the beginning of the headaches. After you overcome the compatibility issure, you have to think about spam. Will your mass email pass the spam filter is another tricky topic.